1. Field of the Invention
The present invention relates generally to monitoring of the wavelength of an optical signal, and more particularly to an optical device for wavelength monitoring and wavelength control.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for directly amplifying signal light has been developed.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In operating the system adopting WDM, the wavelength of each optical signal is monitored to maintain the wavelength locations of the WDM signal light constant. On the basis of the result of the wavelength monitoring, the wavelength of each optical signal is supervised or controlled. Also in a system using a single channel of optical signal, the wavelength of the optical signal is monitored for the purpose of suppressing a chromatic dispersion occurring in an optical fiber transmission line to an allowable small value, for example.
As a device for wavelength monitoring, an optical wavelength detector described in Japanese Utility Model Publication No. 61-22250, for example, is known. This optical wavelength detector includes a beam splitter for extracting first and second branch light beams from a main optical path, a low-pass filter and a high-pass filter for inputting the first and second branch light beams, respectively, first and second photodetectors for receiving output light beams from the low-pass filter and the high-pass filter, respectively, and a differential amplifier for comparing outputs from the first and second photodetectors. By setting the wavelength characteristics of transmittances of the low-pass filter and the high-pass filter so that the transmittances are changed in opposite directions with a wavelength change in the vicinity of a given wavelength, the wavelength is reflected by an output from the differential amplifier, thereby allowing wavelength monitoring.
In the above-mentioned optical wavelength detector in the prior art, two optical filters (the low-pass filter and the high-pass filter) having different wavelength characteristics are required, so that there arises a problem such that the number of parts is increased and the configuration tends to become complicated.